This invention relates generally to gas turbine engines of the fluid-cooled variety and, more particularly, to an improved windage shield for minimizing the temperature rise associated with protrusions in the coolant flow stream.
It is well understood that gas turbine engine shaft horsepower and specific fuel consumption (which is the rate of fuel consumption per unit of power output) can be improved by increasing turbine inlet temperatures. However, current turbines are limited in inlet temperature by the physical properties of their components. Accordingly, many gas turbine engines currently employ fluid cooling to reduce the temperature of the turbine components which are exposed to the hot gases of combustion, thereby prolonging component life. The coolant is typically air which is bled from the compressor or fan section, routed to the turbine and circulated through the turbine disks and blades to effect cooling thereof in a manner well understood by those skilled in this art. In order to take advantage of the potential performance improvements associated with higher turbine inlet temperatures, modern fluid-cooling techniques permit operation at turbine inlet temperatures in excess of 2000.degree. F. (1094.degree. C.). U.S. Pat. Nos. 3,700,348 and 3,715,170, assigned to the assignee of the present invention, are excellent examples of this advanced turbine air-cooling technology incorporating impingement and film cooling.
However, the benefits obtained from such sophisticated air-cooling techniques are at least partially offset by the extraction by the necessary cooling air from the propulsive cycle and, in particular, from the compressor portion. This air which is bled from the compressor and used as a coolant for the turbine has had work done on it by the compressor. However, because it is normally reintroduced into the flow path downstream of the turbine nozzle, it does not return its full measure of work to the cycle as it expands through the turbine. The greater the amount of cooling air which is routed through the turbine, the greater the losses become on the propulsive cycle. Thus, while turbine blade cooling has inherent advantages, it also has associated therewith certain disadvantages which are functions of the quantity of cooling air used in cooling the turbine.
It will also be recognized that the quantity of cooling air required is a direct function of the temperature of the coolant: the lower its temperature, the less that is required. Many gas turbine engines embody a system for routing coolant from the compressor section to the turbine section in which the coolant is metered through at least one rotating seal. To obtain the regulated amount of seal flow (coolant) with minimum performance degradation, the seal is designed to operate with minimal running clearances. These tight clearances produce a temperature rise in the air passing through the seal so that the air has already lost some of its useful cooling capacity prior to reaching the turbine. This situation is aggrevated even further by the presence of protrusions (such as bolts and nuts) which function as turbulence generators, tripping and churning the air in the coolant passage with an attendant increase in temperature level. In particular, often the rotating turbomachinery is fabricated of several components which are connected together at radially inwardly extending mating flanges by at least one circle of axially extending bolts. The source of temperature rise is the relative motion of air impinging on the bolts producing churning of the air. Previous attempts have been made to shield the coolant from such protrusions, but these attempts have not been entirely satisfactory since even the shields have produced some turbulence in the coolant.
It will, therefore, be appreciated that engine performance can be increased by reducing the amount of cooling air required by the turbine and that one means for reducing the quantity is by reducing the temperature rise in the cooling air due to turbulence generators so that a lesser quantity of cooling air can perform the same degree of cooling. Conversely, an increase in turbine component life can be achieved by maintaining the original coolant flow rate but by reducing any temperature rises associated with turbulence generators in routing the coolant to the turbine, with essentially no further degradation in engine performance.